Chlorine is an important basic chemical material which has been widely used in the industries of novel materials such as polyurethanes, silicons, epoxy resins, chlorinated rubbers, chlorinated polymers, chlorinated hydrocarbons and the like; the new energy industries such as manufacture of polycrystalline silicon and the like; the industries of fine chemicals such as disinfectors, detergents, food additives, cosmetic additives and the like; the industries of pesticides/pharmaceuticals such as synthetic glycerin, chlorobenzenes, chloroacetic acid, benzyl chloride, PCl3 and the like; as well as the industries of paper manufacture, textile industries, metallurgy industries and petroleum and chemical industries, etc.
Almost all chlorine is produced by the electrolysis of sodium chloride solution in the industries. This process has two big problems. The first one is the high electricity consumption of up to 2760 kWh per ton chlorine, that makes the electricity consumption of the entire chlor-alkali industry comprises about 5% of the total industrial electricity consumption in China. The second one is the process co-produces chlorine and sodium hydroxide. While when sodium hydroxide requirements do not coincide with the demand for chlorine which increases greatly due to the rapid development of chlorine-consuming industries, oversupply of sodium hydroxide occurs. Thus, it is necessary to find a new source of chlorine for the further development of chlorine-consuming industries.
On the other hand, since chlorine is used as a reaction medium in most chlorine-consuming industries, it is not part of the final products but discharged from reaction systems in a form of hydrogen chloride as a by-product. As the rapid development of chlorine-consuming industries, it is increasingly difficult to find outlets for hydrogen chloride. The resulting by-produced hydrochloric acid has low added value, needs high cost for transport and storage and the sale is difficult. Also, 20-50 times of waste water produced in subsequent applications generates a great deal of pressure on the environment. In the case of co-production of PVC, the domestic capacity of PVC is much excessive, and the export amount, price and utilization of capacity are always unsatisfied. Thus, under the current conditions, the outlet of hydrogen chloride has become a bottleneck restricting further development of the chlorine-consuming industries.
If the by-produced hydrogen chloride could be directly transformed into chlorine, the closed circulation of “chlorine”would be realized, thereby the two bottlenecks of upstream and downstream of the chlorine-consuming industries can be essentially solved. The oxidation of hydrogen chloride by oxygen or air as an oxidant to prepare chlorine is a good route. This reaction is represented by the following stoichiometric formula:2HCl+½O2↔Cl2+H2O−57.7 kJ/mol
Currently, there are three different routes to carry out this process, which are the catalytic oxidation method, the cyclic oxidation method and the oxidative electrolysis method. Among them, the representative cyclic oxidation method is developed by Dupont. In this method, sulfuric acid is used as a cyclic oxidative medium and nitric acid is used as a catalyst. Thus, its equipment investment and operational cost are high, and its operation is complex and lack of flexibility. The oxidative electrolysis method can well relief the second problem, which was describe above, in the chlor-alkali industry. However, it still has an electricity consumption level of above 1700 kWh per ton chlorine, and thereby the status of high electricity-consumption in the production of chlorine is not substantially improved. Furthermore, in comparison to ion-membrane electrolysis, the method of oxidative electrolysis of hydrochloric acid requires more complex equipments and has no advantages in economical efficiency and operability. This technique is mastered only by Bayer. However, Bayer introduced the catalytic oxidation technique from Sumitomo (Japan) while is actively finding a market for its oxidative electrolysis technique.
Objectively, the method of catalytic oxidation of hydrogen chloride also requires relatively large equipment investment, and in general, the cost for production of chlorine is estimated to be slightly higher than that of the method of ion membrane electrolysis according to the present technique of Sumitomo (Japan). The greatest advantage of this method is its low electricity consumption of only about 230 kWh per ton chlorine. In addition, it is an environment-friendly chemical process.
In the reported catalysts for hydrogen chloride oxidation, the active ingredients mainly are metal elements such as copper, chromium, gold and ruthenium, etc. Among them, gold and ruthenium-based catalysts are expensive and have poor performance in sulfur-tolerance. Chromium-based catalysts pollute the environment due to their higher toxicity. Thus, the above two kinds of catalysts have such problems of high economic cost or environmental pollution or the like in use. Compared with them, copper-based catalysts have both advantages of lower cost and being environmentally friendly, thus are of great interests.
CN200710121298.1 discloses a catalyst containing cupric chloride, potassium chloride and cerium chloride with alumina as support and treated by phosphoric acid. For this catalyst the yield of chlorine is 80.1% under the conditions that the ratio of hydrogen chloride and oxygen is 1:1, the temperature of fixed bed reactor is 400°C., the reaction pressure is 0.1 MPa and the space velocity of hydrogen chloride is 0.8 hr−1. However, this catalyst has a relatively low activity, and the loss of the cupric chloride ingredient under a higher temperature impairs the use life of the catalyst.
CN200910027312.0 discloses a catalyst containing cupric chloride, potassium chloride, manganese nitrate and cerium nitrate supported on silica gel or ReY molecular sieve. With 25 g of this catalyst, the hydrogen chloride conversion is 83.6% with both of hydrogen chloride and oxygen flow rates of 200 ml/min at a reaction temperature of 380°C. However, this catalyst still has the disadvantages of loss of copper ingredients and a relatively low space velocity.
U.S. Pat. No. 4,123,389 discloses a copper-based catalyst with silica gel, alumina or titania as a support, in which the loading amount of active ingredients is between 25% and 70%. The process of preparation of the catalyst needs organic solvents and thus causes great environmental pollution.
Therefore, it is still a technical challenge in the related field to develop a cheap, environment-friendly catalyst with high activity and stability for production of chlorine by catalytic oxidation of hydrogen chloride.